It is frequently desirable to dim lights. Dimmers are found in many residential and commercial locations. This dimming has been done historically by removing power from the light during a certain portion of each line cycle, as is accomplished using a triac dimmer. However, it is becoming increasingly common for the dimming level to be set by a DC signal, typically 0-10V.
Traditionally, dimmers have been used with incandescent light bulbs. These bulbs have very particular characteristics as they dim. Not only does the light they produce get dimmer, it also becomes redder, so-called ‘warm-on-dim’. One measure of the change in color of the light is the CCT. The CCT of a light is measured by finding the closest black-body radiation emission temperature of the light, and so is measured in degrees Kelvin (K). A typical incandescent light bulb, as it is dimmed, changes from about 2800K to about 1800-2000K. The relationship between dim level, measured by average voltage applied over a line cycle, and CCT is highly non-linear.
An increasingly common form of lighting is the LED. These work by applying a DC current through them. The amount of light they emit is approximately proportional to the magnitude of the current. It is thus common to dim LEDs by controlling their current as a function of the dim level, which is determined by either the average or RMS voltage, or by the DC 0-10V signal.
However, a problem arises when dimming LED lights. A particularly common form of LED is the so-called ‘white’ LED. These can be designed to have a CCT anywhere along the black-body curve, from 6000K down to below 2500K. However, for a given LED their CCT is basically not adjustable. When the current through them is reduced, they produce less light, but the CCT is unaffected. White LEDs thus do not produce the same type of light output on a dimmer as does an incandescent light bulb.
The typical method of solution to this problem in an LED light is to use a variety of different colored LEDs, such as a combination of red, green and blue (RGB), and to mix their light emissions together optically. The amount of current in each may be designed to produce white light of a particular CCT, for example that of an incandescent bulb at full brightness. When used with a dimmer, the ratios of currents in the different colors is varied in such a way as to produce a desired arbitrary CCT, and in particular, one that is close to that of an incandescent being similarly dimmed.
However, generating and controlling the currents appropriately to the three colors of LEDs typically involves considerable circuitry, and frequently is done under software control of a microcontroller that is built in to the light. This level of complexity makes such a control expensive and large. Furthermore, the type of red LED used to achieve this performance has a very significant change in light output as a function of its temperature, even at constant current drive level. To compensate for this, the microcontroller will typically also sense the temperature of the light, applying corrections to the amount of current received by the red LED as a function of temperature.
Yet further, the red LEDs used also have a very significant drop-off in light output with age. After a few years of operation, their light output has become significantly reduced, and the amount of that reduction is different than that of the green or blue LEDs used together with the red to produce the white light. To compensate for this, a light sensor is sometimes built in to the light to detect the light's spectral characteristics, and adjust the red LEDs' current to compensate for this aging. The light sensor can also be used to adjust the red LEDs' current to achieve the desired CCT as a function of dim level. The light sensor further adds cost and size to the design of the light, and the temperature measurement and correction of the red LEDs' drive current further add to the complexity of the firmware used by the microcontroller.
It would be desirable to have an LED system that could be dimmed in such a way as to emulate the light output of an incandescent bulb, both in brightness and in CCT, when operated on either a triac or 0-10V dimmer. It would be desirable that it would be insensitive to temperature variation and aging, and that it would be easy to control without the use of a microcontroller or a spectral sensor. It would also be desirable that it would be inexpensive, and that it not require the use of three or more different types of LEDs.